The surfaces of steel sheets may be galvanized by a method such as electrogalvanization and hot-dip galvanization so as to improve the corrosion resistance and durability of such steel sheets. As compared with non-coated steel sheets, galvanized steel sheets have high corrosion resistance owing to zinc functioning as a sacrificial anode and are thus widely used as materials for automobiles, home appliances, and constructions.
However, in such industrial fields, coated steel sheets being relatively lightweight and highly economical, in addition to having higher degrees of durability, are increasingly required.
Meanwhile, due to the limited amount of zinc deposits in the earth and concern about the depletion of zinc reserves, the trend of using zinc deposits as a weapon has increased. Moreover, existing electrogalvanizing processes and hot-dip galvanization processes consume a large amount of energy and emit pollutants, disadvantageous, in that regulations on pollutants are being strengthened worldwide.
Due to these reasons, Zn—Mg alloy-coated steel sheets formed by a vacuum deposition method have received attention. According to the vacuum deposition method, a coating process is performed in a vacuum to produce Zn—Mg alloy-coated steel sheets. Therefore, the emission of pollutants is very low, and production rates higher than those of existing plating processes may be obtained according to the development of techniques for evaporating coating materials. Moreover, since Zn—Mg alloy-coated steel sheets having high corrosion resistance and thin coating film thicknesses can be manufactured, Zn—Mg alloy-coated steel sheets have received attention as relatively economical future steel sheets.
Although Zn—Mg coatings have such merits in terms of manufacturing processes and product properties, Zn—Mg coatings are harder and more fragile than zinc coatings, and thus it is difficult to improve the adhesion of Zn—Mg coatings.
Furthermore, since magnesium (Mg) is very active, the surface of a product coated with Zn—Mg may react with moisture in humid environments, and thus the surface of the product may be blackened and worsened in quality.
Patent Document 1 discloses a technique for solving such a blackening problem. According to the technique disclosed in Patent Document 1, blackening of a Zn—Mg coated product is prevented by treating the Zn—Mg coated product with a 0.01% to 30% calcium phosphate solution so as to elute magnesium (Mg) and adjust the content of magnesium (Mg) in the surface of the Zn—Mg coated product to be 1 wt. % or less. However, according to the technique disclosed in Patent Document 1, magnesium (Mg) is not uniformly eluted. Thus, the surface of the Zn—Mg coated product may be stained or may be blackened due to the formation of calcium phosphate including magnesium (Mg) during a calcium phosphate solution treatment.
(Patent Document 1) Japanese Patent Application Laid-open Publication No. 1997-2418